Photodetectors are broadly defined as devices which respond to incident electromagnetic radiation, thereby enabling measurement of the intensity of the incident radiation. A photodetector typically includes some sort of photoconductive device and external measurement circuitry.
The detection of ultraviolet (UV) light under daylight conditions is an important problem for commercial (household and professional) and defence applications. Photodetectors with sensitivity in the UV have wide applicability. Exposure to UV-A and UV-B radiation (320-400 nm and 280-320 nm, respectively) can lead to skin cancer, making quantification of UV-A and UV-B light important for health reasons. Detection in the UV-C range (from 200-280 nm), which is termed “deep UV” (DUV), is important in solar radiometry, scientific research (such as in scintillation detectors), environmental studies, and biological research. UV-C photosensors have commercial application in fire alarms, combustion monitoring, missile plume detection, and space-to-space transmissions (Peng et al., Adv. Mater. 2013, doi: 10.1002/adma.201301802). In contrast to UV-A and UV-B light, UV-C light from the sun is completely absorbed by the earth's atmosphere and therefore does not interfere with DUV reporting.
A particular challenge is the design of “solar blind” detectors that are sensitive to very low levels of UV-C radiation and insensitive to visible light. Since the solar spectrum ceases at about a wavelength λ of 290 nm, a solar-blind detector shall be defined as a device or apparatus that only responds to wavelengths below 285 nm.
A variety of fabrication routes for UV photodetectors have been explored, ranging from vacuum-based methods such as epitaxial growth of thin films and vapor deposition of nanowires to solution-processing, in the form of sol-gels, nanocolloidal inks, and spray pyrolysis. Ga2O3, for example, can be made using sputtering, chemical vapor deposition, CVD, pulsed laser deposition, spray pyrolysis, and sol-gel methods (Appl. Phys. Lett. 90, 031912, 2007). Each approach boasts certain achievements, but also presents challenges.
One group of devices are the UV photodetectors based on single crystalline absorbers. Narrow bandgap materials such as silicon and III-V compounds may be used for UV photodetection; however, their spectral range must be modified through the use of high-pass optical filters or by the incorporation of phosphors. To further reduce the dark current, such devices are typically cooled during operation. Over the long-term, exposure to irradiation of higher energy the band-gap may damage the active material (Peng et al., Adv. Mater. 2013, 35, 5321-5328).
To avoid these issues, epitaxial growth of wide band-gap semiconductors with intrinsic “visible blindness” have been explored for use in UV photodectors. Photodetectors based on crystalline MgxZn1-xO (4.76 eV, bandgaps provided in brackets), InGeO and ZnGeO (4.43 and 4.68 eV), β-Ga2O3 (4.8 eV), AlxGa1-xN (>3.4 eV), AlN, BN, and diamond have been reported. A challenge of this approach lies in growing high quality, lattice-matched films on low-cost substrates. Often, the resulting material suffers from a large density of dislocations and grain boundaries. The photoconductivity depends on stoichiometry and, in the case of the metal oxides, on gas absorption phenomena. However, the ability to tune the bandgap through doping is limited by the tolerance of the crystal lattice. Many polycrystalline films exhibit slow response times ranging from a few minutes to several hours (Jin et al., Nano Lett., Vol. 8, No. 6, 2008).
Another group of devices are the UV photodetectors based on nanostructured crystalline absorbers. In this area, two different approaches can in principle be used, in which inks contain either molecular precursors that decompose to form the target material, or preformed, crystalline nanoparticles.
Nanostructured UV photodetectors may have advantages compared to those based on bulk materials. Carrier confinement can lead to higher responsivity and increased photoconductivity gains. For metal oxides, the high surface area to volume ratio facilitates gas adsorption and desorption, which may suppress the dark current (Peng et al., Adv. Mater. 2013, 35, 5321-5328).
Nanowires of a number of binary and ternary metal oxides have been incorporated into such UV photosensors, including Nb2O5 nanobelts for UV-A sensing (Adv. Funct. Mater. 2011, 21, 3907-3915) and Zn2GeO4 and In2Ge2O7 nanowires (J. Mater. Chem. C, 2013, 1, 131-137). Devices based on bridged assemblies of ZnO or Ga2O3 nanowires can be fabricated in a single chemical vapour deposition step (Li et al., Adv. Funct. Mater. 2010, 20, 3972). Devices based on a single Ga2O3 nanobelt grown using chemical vapor deposition exhibited high selectivity towards 250 nm light, fast response times of less than 0.3 s, and a S/N ratio greater than 4 orders of magnitude (Li et al., Nanoscale, 2011, 3, 1120).
High performing UV photodetectors have also been demonstrated based on inks comprised of pre-formed crystalline nanostructures. Jin et al. describe (Nano Lett., Vol. 8, No. 6, 2008) the fabrication of a solution-processed photodetector through spin-coating colloidal ZnO nanoparticles and annealing the thin film in air. The devices exhibit high UV photocurrent efficiencies with a responsivity of 61 A/W of 370 nm light and low dark currents with a resistance >1 TΩ. Notably, the response time for these materials is quite rapid, under 0.1 s and about 1 s for the rise and fall, respectively. Photodetectors active in the near UV have been demonstrated using In2O3 nanoparticles (Shao et al., App Surface Science 261 (2012) 123)
Precursor Based Approach:
Metal oxides thin films can readily be generated using sol-gels. Comparisons of the sol-gel films to vacuum deposited films indicate that the solution-processed devices may have improved performance (J. Vac. Sci. Technol. B 30, 031206, 2012). By tailoring the annealing conditions after sol-gel deposition, a variety of nanostructures can be accessed, including vertically aligned ZnO NWs (Bai et al., Current Applied Physics 13 (2013) 165e169)
Ga2O3 deep UV photodetectors have also been reported using sol gel methods. In one case, Ga2O3 was prepared using gallium isopropoxide as the precursor and methoxyethanol and monoethanolamine as the solvent and stabilizer, respectively. The films were annealed at temperatures ranging from 400-1200° C. A spectral response was observed for films heated to 600° C. and above, with the peak value of the photocurrent increasing with heat-treatment temperature up to 1000° C. (Appl. Phys. Lett. 90, 031912, 2007). In another study, Ga2O3 devices produced using a sol-gel had high responsivity of over 1 A/W (Appl. Phys. Lett. 98, 131114, 2011). The documents JP 2008282881 a and JP 2009044019 A likewise report such sol-gel approaches for making indium oxide containing films.
Spray pyrolysis is a viable alternative to sol gel processing. Photoconductive detectors based on Ga-doped ZnO deposited through spray pyrolysis at 450° C. have been reported (Shinde and Rajpure, Mat. Res. Bull., 46 (2011) 1734). Under illumination at 365 nm (2 mW/cm2), a current over 2 mA was generated.
Ga2O3 nanoparticles have also successfully been synthesized using spray pyrolysis of gallium nitrate. In this case, Ga(NO3)3 was dissolved in ultra pure water and combined with lithium chloride as a flux salt. The resulting solution was atomized and transferred as a mist into an alumina reactor (700-1000° C.) to form Ga2O3 nanoparticles as a route toward GaN nanoparticles. A photodetector was not fabricated (Ogi et al., Advanced Powder Technology 20 (2009) 29-34). A lower temperature process was reported by Kim and Kim (J. Appl. Phys. 62 (5), 1987), in which GaCl3 was sprayed from aqueous solvent onto a substrate heated to 350° C. While a device was not made in this instance, XRD data matched that of Ga2O3, and optical measurements indicated a bandgap of 4.23 eV.
A technological need exists for a material platform for UV detectors that is compatible with low-temperature processing and large-area integration. Processing routes are sought which have the potential to lower costs and allow for flexible device architectures. As of yet, a low temperature (<500° C.) route to deep UV photodetectors (responsive below 280 nm) that are truly solar blind is still needed.